Micro relief element and preparation thereof

ABSTRACT

A micro relief element which comprises  
     a) a first layer of a first substrate, the first layer having a receptive surface capable of retaining a relief forming polymer;  
     (b) an overlay of a desired thickness of the relief forming polymer over the receptive surface; and  
     (c) at least one relief feature formed from the relief forming polymer and which protrudes above the overlay; structures and elements comprising such micro relief element; micro-optical, micro-fluidic, micro-electrical and micro-chemical applications thereof; and a method and apparatus for the preparation thereof.

[0001] This invention relates to a micro relief element (MRE) and amethod of preparing same.

[0002] An MRE, as referred to herein, is a 3-dimensional structure whichis formed on the surface of a desired substrate and which structure isable to perform a specific function. Typically, the structure is arepetitive pattern which protrudes above the substrate to a definedheight of the order of 0.1 to 1000 microns. Such an MRE can be used asan active component in micro-optic, micro-fluidic, micro-electrical andmicro-mechanical devices. In particular, such an MRE can be used as amicro-optical element (MOE) and in which case the structure may be of aheight in the range 0.1 to 1000 microns, more commonly in the range 0.1to 10 microns. Where the MRE is a component in a micro-fluidic ormicro-mechanical device then the structures are usually of heights inthe range 10 to 1000 microns.

[0003] An MOE comprises a surface relief structure whose purpose is toinduce phase changes on a light beam which is incident upon thestructure such that a predetermined spatial distribution of the lightresults when the incident light is viewed either in reflection ortransmission. MOEs also include structures in which the relief structureis embedded within a light transmissive material, hereinafter animmersed MOE, such as for example an immersed microlens.

[0004] MOEs may be used for a variety of applications, such asdiffraction gratings, lenses, beam array generators, laser harmonicseparators, focusing mirrors and microlens arrays.

[0005] Microlens arrays can be used for optical readers, interfacesbetween laser diodes and optical fibres, diffuser screens, integralphotography, 3-d camera and display systems, integrated optical devicesand imagebars.

[0006] Usually, an MOE is formed by exposing and developing the desiredsurface relief structure into a photosensitive material coated onto thesupporting substrate and then transfering the surface relief structureinto the substrate by plasma or chemical etching. The conventionaldesign and fabrication of MOEs is discussed in “Synthetic diffractiveelements for optical interconnects”, M R Taghizadeh et al, OpticalComputing and Processing, Vol 2(4), pp 221-242, 1992; “Two-dimensionalarray of diffractive microlenses fabricated by thin film deposition”, JJahns et al, Appl Opt, Vol 29(7), 931, 1990; “Continuous-reliefdiffractive optical elements for two-dimensional array generation”, M TGale et al, Appl Opt, Vol 32(14), 2526, 1993; “Multilevel-grating arraygenerators: fabrication error analysis and experiments”, J M Miller etal, Appl Opt, Vol 32(14), 2519, 1993; and “Fabricating binary optics ininfrared and visible materials” M B Stem et al, SPIE, Vol 1751,Miniature and micro-optics, pp 85-95, 1992.

[0007] Microlens arrays have in the past been produced by differentmethods as described in “Polymer microlens arrays”, P Pantelis and D JMcCartney, Pure Appl. Opt., Vol 3, 103 (1994); “The manufacture ofmicrolenses by melting photoresist”, D Daley, R F Stevens, M C Hutleyand N Davies, Meas. Sci. Technol., Vol 1, 759 (1990); and “Microlensarray fabricated in surface relief with high numerical aperture”, H WLau, N Davies, M McCormick, SPIE Vol 1544 Miniature and Micro-optics:Fabrication and System Applications, p178 (1991). Glass microlenses havebeen made by chemically etching glass, moulding glass, plasma etchingglass to produce a surface relief structure. Polymer microlenses havebeen produced by melting islands of photoresist or by direct writingphotosensitive materials with a laser beam or by directly writing asuitable material with an electron beam or by plasma etching or bymoulding.

[0008] Unfortunately, conventional methods of fabrication for MREs arelimited in the range of substrates that can be used and in thecomplexity and accuracy of the relief structures that can be formed.

[0009] It is an object of the present invention to provide a facilemethod for producing MREs, in particular MOEs, in a variety ofsubstrates and complexity of designs. An advantage of the present methodis that a wide range of heights of surface relief can be produced usingthe same process. Another advantage is that small lateral features canbe successfully reproduced. Additionally, the process may be used toproduce large area MREs.

[0010] Accordingly in a first aspect the present invention provides amicro relief element which comprises

[0011] a) a first layer of a first substrate, the first layer having areceptive surface capable of retaining a relief forming polymer;

[0012] (b) an overlay of a desired thickness of the relief formingpolymer over the receptive surface; and

[0013] (c) at least one relief feature formed from the relief formingpolymer and which protrudes above the overlay.

[0014] In a second aspect the present invention provides a structure foruse as at least part of a micro-optical element, which structurecomprises

[0015] (a) a first layer of an optically transmissive first substratehaving a first refractive index, the first layer having a receptivesurface capable of retaining an optically transmissive relief formingpolymer;

[0016] (b) an overlay having an optically insignificant effect,preferably having a maximum thickness of less than 1.5 m, of the reliefforming polymer over the receptive surface, the relief forming polymerhaving a second refractive index which is the same as or different fromthe first refractive index; and

[0017] (c) at least one optically active relief feature formed from therelief forming polymer and which protrudes above the overlay.

[0018] In a third aspect of the present invention there is provided animmersed MOE comprising

[0019] 3(a) a first layer of an optically transmissive first substratehaving a first refractive index, the first layer having a receptivesurface capable of retaining an optically transmissive relief formingpolymer;

[0020] (b) an overlay having an optically insignificant effect,preferably having a maximum thickness of less than 1.5 m, of the reliefforming polymer over the receptive surface, the relief forming opticallytransmissive polymer having a second refractive index which is the sameas or different from the first refractive index;

[0021] (c) at least one optically active relief feature formed from therelief forming polymer and which protrudes above the overlay; and

[0022] (d) a second layer of an optically transmissive second substratehaving a third refractive index which is superimposed upon the at leastone optically active relief feature and wherein not all of the first,second and third refractive indices are the same.

[0023] In a fourth aspect of the present invention there is provided amethod of preparing a micro relief element which comprises

[0024] a) a first layer of a first substrate, the first layer having areceptive surface capable of retaining a relief forming polymer;

[0025] (b) an overlay of a desired thickness of the relief formingpolymer over the receptive surface; and

[0026] (c) at least one relief feature formed from the relief formingpolymer and which protrudes above the overlay

[0027] which method comprises

[0028] (a) forming a line of contact between the receptive surface andat least one mould feature formed in a flexible dispensing layer;

[0029] (b) applying sufficient of a resin, capable of being cured toform the relief forming polymer, to substantially fill the at least onemould feature, along the line of contact;

[0030] (c) progressively contacting the receptive surface with theflexible dispensing layer such that

[0031] (1) the line of contact moves across the receptive surface;

[0032] (2) sufficient of the resin is captured by the mould feature soas to substantially fill the mould feature; and

[0033] (3) no more than a quantity of resin capable of forming theoverlay passes the line of contact;

[0034] (d) curing the resin filling the at least one mould feature so asto form the at least one relief feature; and, optionally, thereafter

[0035] (e) releasing the flexible dispensing layer from the at least onerelief feature.

[0036] In a fifth aspect of the present invention there is provided amethod of preparing a structure for use as at least part of amicro-optical element, which structure comprises

[0037] (a) a first layer of an optically transmissive first substratehaving a first refractive index, the first layer having a receptivesurface capable of retaining an optically transmissive relief formingpolymer;

[0038] (b) an overlay having an optically insignificant effect,preferably having a maximum thickness of less than 1.5 m, of the reliefforming polymer over the receptive surface, the relief forming polymerhaving a second refractive index which is the same as or different fromthe first refractive index; and

[0039] (c) at least one optically active relief feature formed from therelief forming polymer and which protrudes above the overlay

[0040] which method comprises

[0041] (a) forming a line of contact between the receptive surface andat least one mould feature formed in a flexible dispensing layer;

[0042] (b) applying sufficient of a resin, capable of being cured toform the relief forming polymer, to substantially fill the at least onemould feature, along the line of contact;

[0043] (c) progressively contacting the receptive surface with theflexible dispensing layer such that

[0044] (1) the line of contact moves across the receptive surface;

[0045] (2) sufficient of the resin is captured by the mould feature soas to substantially fill the mould feature; and

[0046] (3) no more than a quantity of resin capable of forming theoverlay passes the line of contact;

[0047] (d) curing the resin filling the at least one mould feature so asto form the at least one optically active relief feature; and,optionally, thereafter

[0048] (e) releasing the flexible dispensing layer from the at least oneoptically active relief feature.

[0049] An MRE of the present invention may be capable of use as anactive component in a micro-optic, micro-fluidic, micro-electrical ormicro-mechanical device. However, the principle use herein envisaged foran MRE of the present invention is as a micro-optical element (MOE).Reference herein to features making up an MOE according to the inventionmay be to features which are equally advantageous in other applicationsof MRE's and references to MOE's will be construed as referring to MRE'saccordingly.

[0050] Such an MOE may be able to perform more than one opticalfunction, e.g. an MOE for use as a beam corrective optic for diodelasers may combine the functions of astigmatism correction, elipticitycorrection and beam collimation.

[0051] Moreover, the optically active relief feature in combination withthe supporting first layer may be able to perform more than one opticalfunction, for example an optically active relief feature supported on ashaped first layer, suitably of lens shape, may provide for correctionof chromatic aberration.

[0052] Accordingly it will be apparent that the first layer and indeedthe MRE or MOE, and the relief feature(s) may be of any desired geometryaccording to the desired function to be performed. For example the firstlayer, including an optional support substrate, may be planar, hollow orsolid cylindrical, or may comprise a lens or other optical componentwherein the relief feature(s) is/are suitably applied to a surfacethereof. Alternatively or additionally the relief feature(s) may forexample comprise one or more continuous, stepped or otherwise profiledstructures such as lens, straight or angled track or lateral, annularring, straight or curved diffraction grating, multiple faced(pyramidal), or other optical, fluidic, electrical or mechanicalstructure.

[0053] Additionally, the MOE may be coated with an other material inorder to protect the MOE (anti-scratch coating) or to reduce reflectionfrom the MOE (anti-reflection coating). Preferably, such coatings aremultilayered coatings.

[0054] Furthermore, the MOE may function in reflection rather thantransmission. This might be achieved by fabricating the MOE using areflective first layer or by coating the surface of the MOE to enhancereflection from it.

[0055] The first layer may be supported by a suitable support substratewhich may be subsequently removed from the first layer. However, it ispreferred that the first layer is self-supporting or is associated witha support surface of desired geometry for a desired application.Suitably the first layer is comprised of any suitable material for theintended application which may be known in the art for example it may bea polymer film (in particular a film formed from polyester, such as PETor PEN, or an other polymer such as PVC, polyimide, PE or a knownbiodegradable polymer, e.g. poly(hydroxy butyrate)); a material selectedfor its optical transparency at certain wavelengths for example ZnSe orGermanium which are capable of operation in the infra-red region between2 and 15 micron; silicon; high temperature resistant inorganic metaloxide or ceramic such as titania or (fused) silica, e.g. glass; or itmay be a natural or synthetic paper product such as a wood pulp orsynthetic card or paper.

[0056] For certain applications, for example where semiconductorcomponents are mounted onto the MRE and from which it is desirable todissipate heat, the first layer may be coated with a layer of diamond orsimilar material with a high thermal conductivity.

[0057] Additionally, the first layer may be coated with an electricallyconducting layer, e.g. indium tin oxide (ITO) or gold, so that anelectrical contact can be made to a semiconductor component located onthe surface of the first layer.

[0058] The receptive surface of the first layer may be coated with asuitable bonding agent, e.g. where the first layer is of glass, a silanecoupling agent, which serves to more firmly anchor the relief feature tothe first layer.

[0059] Coating of the first layer may be achieved as a continuous layerprior to forming the optically active relief structure(s) thereon, butis advantageously achieved as a layer about the optically active reliefstructure(s), which may be created by replication from the flexibledispensing layer during the formation of the optically active reliefstructure(s).

[0060] The second layer may also be supported by a suitable, optionallyreleasable, substrate. The second layer may be superimposed on the atleast one optically active relief feature by any suitable means, e.g.lamination. The second layer may also be provided with at least onemould feature in which is moulded an optically transmissive polymer,which may be the same as the optically transmissive relief formingpolymer retained on the receptive surface, and which may be so placedthat at least some of the mould features of the second layer are matchedwith at least some of the mould features of the first layer such thatthey can form a composite optical component. The selection of the reliefforming polymer will be dependent on the intended use of the MRE andincludes silica filled, light curable resins such as those used indentistry and those for rapid prototyping by stereolithography, UVcurable liquid crystal resins, photocationic epoxy resins and thoseoptically transmissive resins as described below.

[0061] When optically transmissive, the relief forming polymer may beselected from those known in the art including those developed as lightcurable adhesives for joining optical components for example those soldunder the name LUXTRAK (LUXTRAK is a tradename of Zeneca plc), thosedeveloped for polymer optical fibre fabrication and those developed foroptical recording using polymer photoresists. In particular theoptically transmissive relief forming polymer may be formed from asuitable resin for example halogenated and deuterated siloxanes,styrenes, imides, acrylates and methacrylates such as ethyleneglycoldimethacrylate, tetrafluoropropylmethacrylate,pentafluorophenylmethacrylate, tetrachloroethylacrylate, multifunctionalderivatives of triazine and phosphazene. Resins and polymers thatcontain highly fluorinated aliphatic and aromatic moieties arepreferred.

[0062] Preferably, the optically transmissive relief forming polymer isselected to have as near as possible equal and opposite thermalexpansion and thermo-optic coefficients. The advantage of this is thatincreases in the optical path length (and hence phase change) due tothermal expansion of the material are compensated by decreases in itsrefractive index. This advantage requires that the optically activerelief is restrained from expanding laterally by the effect of thesubstrate material. This will be the case when the overlayer is small.“Temperature dependence of index of refraction of polymeric waveguides”,R Moshrefzadeh, M D Radcliffe, T C Lee and S K Mohapatra, J LightwaveTech, vol 10 (4), 420 (1992) describes a number of polymer materialshaving negative thermo-optic coefficients, positive thermal expansioncoefficients of the same magnitude. For example, PMMA has a thermo-opticcoefficient of −1.1×10⁻⁴ K⁻¹.

[0063] Preferably, the optically transmissive polymer has a refractiveindex which is matched to the first refractive index, e.g. 1.51 at 633nm when the first layer is Bk7 borosilicate glass or 1.46 at 633 nm whenthe first layer is quartz.

[0064] The refractive index of the optically transmissive relief formingpolymer may be modified by the inclusion of suitable additives into thepolymer. In particular the refractive index of the polymer may beadjusted by adding appropriate amounts of ethylene glycol dimethacrylatewhich can increase the refractive index (as measured at 1.32 or 1.55 m)by an absolute value in excess of 0.02 when added at a level of 30% byweight.

[0065] Furthermore, an error in the depth of the optically active relieffeatures (compared to the designed depth) can be corrected by increasingor decreasing the refractive index of the optically transmissive reliefforming polymer by an equal fractional amount.

[0066] A further advantage of controlling the refractive index of theoptically transmissive relief forming polymer is that the wavelength ofoperation of the MOE is shifted as a result. Hence a series of MOEs canbe produced from the same flexible dispensing layer so as to obtain anMOE which operates at high efficiency at the chosen wavelength. Changingthe refractive index from 1.45 to 1.55 for an MOE designed to operate at633 nm for example would result in maximum efficiency operation at 677nm.

[0067] The overlay of the relief forming polymer is reproduciblycontrolled to obtain a thickness appropriate to the function of the MREand may, even in those instances where a minimum overlay is desired,usefully serve to planarise the receptive surface. In some instances,e.g. in micro-mechanical devices, a relatively thick and uniform overlaymay be desirable for example to secure the relief forming polymer firmlyto the first layer. In other instances, e.g. where the MRE is an MOE, itis desirable to minimise the thickness of the overlay such that it doesnot interfere significantly with the optical function of the MOE, i.e.the overlay is optically insignificant. Preferably, the opticallyinsignificant overlay has a maximum thickness of less than 1.5 m,preferably less than 1 m, and particularly less than 0.5 m over thesurface of the first substrate. The average thickness of the opticallyinsignificant overlay is preferably less than 1 m and particularly lessthan 0.5 m. The variation of the thickness of the overlay, whetheroptically insignificant or not, across the surface is preferably lessthan ±0.75 m, particularly less than ±0.5 m and especially less than±0.25 m. This has the particular advantage of minimising wavefronterror.

[0068] The optical performance of the MOE depends on the phasedifference produced between parts of the light beam which travel throughdifferent areas of the surface relief pattern. The phase difference isdefined by the product of the depth of the features below the surface ofthe MOE and the refractive index of the material in which the MOE isproduced. An advantage of having less than 1 micron of overlay betweenthe first layer and the optically active relief is that this height iswell defined. Hence the MOE functions as designed. Also important is theflatness of the intervening surface between the optically active relieffeatures of the MOE. Improved performance results if the interveningsurface is flatter than the wavelength of the light being used. Withminimum overlay, the intervening surface is as flat as the first layeron which it is produced. Another advantage of minimum overlay is that itreduces optical loss of the part resulting from absorption of light bythe material by minimising the total thickness of material required todefine the surface relief pattern.

[0069] A very significant advantage of making polymer optically activerelief features on glass or another material with a low thermalexpansion coefficient is that the thermal stability of the MOE componentis enhanced as a result of maintaining the pitch of such opticallyactive relief features and by minimising the volume of that materialwhich has a relatively high thermal expansion coefficient.

[0070] In order to facilitate the curing of the resin it is preferred touse an initiator, for example a thermal and/or photoinitiator andparticularly an initiator which does not absorb light at the operatingwave length of the MOE. Typically, when used, an initiator is present inthe resin at a concentration from 0.1 to 3.0% by weight, and preferablyfrom 0.5 to 2.0% by weight.

[0071] Suitable photoinitiators include2-methyl-1-[4-(methylthio)phenyl)-2-morpholino propanone-1 (Irgacure907), 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure184),isopropylthioxanthone (Quantacure ITX),Camphorquinone/dimethylaminoethylmethacrylate. Similarly a suitablethermal initiator is tert-butylperoxy-2-ethyl hexanoate (Interox TBPEH).

[0072] As the line of contact moves across the surface of the firstlayer the resin is effectively pushed across the surface and flows intothe at least one mould feature. The rate at which the line of contactadvances across the surface will depend, amongst other things, on thecharacteristics of the resin. Typically, the resin has a viscosity from0.1 to 100 poise and more typically from 10 to 100 poise.

[0073] The resin may be fully retained within a mould feature as theline of contact moves from the mould feature, in which case the resinmay be cured at any convenient subsequent time. However, the resin mayoften show some degree of resilience in the non-cured form in which caseas the line of contact moves from the mould feature the resin thereinwill tend to relax and exude from the mould feature. Where the relieffeature is part of an MOE then this relaxation of the resin can reducethe effectiveness of the MOE. To counter the relaxation of the resin itis preferred that the resin is cured before the line of contactcompletely moves from it.

[0074] Conveniently and preferably therefore, the resin contains aphotoinitiator which is activated by a particular wavelength of light,particularly UV light. A suitable source of light may then be used tocure the resin before the pressure applied along the line of contact isreleased and before the resin relaxes from the retaining feature. It isespecially preferred that the flexible dispensing layer is transparentto the light used and that the light is shone through the flexibledispensing layer towards the resin. In order to focus the lightsubstantially at the tip and thereby avoiding, for example, prematurecuring of the resin, the angle of incidence of the light onto the lineof contact may be required to be adjusted from polymer to polymer.Alternatively, for a given angle of incidence and where the first layeris at least partially transmissive to the light, the first layer may bechosen to have a thickness such that the internal refraction of theincident light acts to focus the light at the line of contact.Additionally, where the first layer is at least partially transmissiveto the light and is of a suitable thickness, a mirrored support may bepositioned under the first layer thereby causing the transmitted lightto be reflected back to the line of contact.

[0075] The pressure is applied along the line of contact by any suitablemeans. Suitably, the pressure is applied using an advancing bar orflexible blade under a compressive load which may be drawn along thesurface, or using a roller under a compressive load which may thus onadvancement or rotation retain the resin in the nip formed by the bar,blade or roller between the flexible dispensing layer and the surface.It is therefore preferred that the resin is cured at the nip as the lineof contact progresses across the surface.

[0076] The flexible dispensing layer is preferably a polymer film intowhich the mould features have been embossed. Such an embossed film ispreferably transparent to UV light, has high quality surface releaseproperties and is capable of remaining dimensionally sound during themoulding process. Conveniently, such an embossed film may be formed by(a) forming a master pattern having a contoured metallised surface whichconforms to the required relief structure, (b) electroforming a layer ofa first metal onto the metallised surface to form a metal master, (c)releasing the metal master from the master pattern, (d) repeating theelectroforming process to form a metal embossing master shim and (e)embossing the relief structure into a polymer film so as to form thedesired mould features.

[0077] Adventitiously, when transparent, the embossed film may beoptically aligned so that the mould features may be precisely aligned onthe receptive surface of the first layer. Thus, the mould features maybe more easily oriented on the receptive surface, e.g. about a desiredaxis of or existing feature on the receptive surface. In particular,where the first layer is itself a lens then the optical axis of the lensmay be aligned with that of an optically active relief feature formedusing the mould features such that the optical performance of thecomposite component is optimised.

[0078] Additionally, the embossed film, if retained on the receptivelayer, may serve as a protective layer which can be removed at a latertime.

[0079] A further advantage of making the MOE by the above method is thatthe refractive index of the relief forming polymer may be varied so asto improve or modify the optical performance of the MOE. This is also abenefit because optical components with different operating wavelengthscan be made from the same master shim.

[0080] A further advantage of making the MOE by the above method is thatthe master pattern can be made by a wide range of available techniquesin a wide range of materials and is not limited to being made in amaterial with good optical properties. For example the original masterpattern can be made by direct electron beam patterning of photoresist,conventional photolithography, silicon micromachining (K E Peterson,Proc IEEE, Vol 70, 420 (1982)), laser beam writing (E C Harvey, P TRumsby, M C Gower, S Mihailov, D Thomas, Excimer lasers forMicromachining, Proc of IEE Colloquium on Microengineering and Optics,February 1994, digest No. 1994/043, paper 1; D W Thomas et al, Laserablation of electronic materials, European Mat Res Soc Monographs, Vol4, Ed. E Fogarassy and S Lazare, p221 (1992); H Schmidt, Micromachiningby lasers, Conf on Lasers and Electro-optics (CLEO EUROPE 94),Amsterdam, September 1994, Paper CMB1); plasma etching (D L Flanmn inPlasma etching—an introduction ed by D M Manos and D L Flarnm, AcademicPress Inc, London (1989), Chapter 2); and single point diamond turning.

[0081] A further advantage of making the MOE by the above method is thatthe flexible dispensing layer may be treated with any suitable materialfor any desired purpose, for example a masking or screening medium, apriming medium, or a medium conferring any desired optical, electrical,mechanical or fluid properties, such as ink, seed (catalyst) material, ametal precursor, an electrically conducting (precursor) medium, or abiological culture or the like which may be transferred by contactreproduction to the first layer or the overlayer as desired, for exampleto selected regions thereof on or about the relief features, using amodification of known techniques for example as described in Appl. Phys.Lett. 68(7), 1022-23

[0082] Moreover microlenses comprising relief features having a widerange of aspect ratios, i.e. of height to width ratio, may be produced,for example of aspect ratio up to 20, suitably up to 10 or up to 15depending on the relief forming polymer and the relief feature shape.

[0083] An advantage of fabricating an MOE in the form of a microlensarray by the above method is that the shape of the surface of each lensis determined by the mould and not by the fabrication process. This isin contrast with the conventional method of producing microlens arrayswhich relies on surface tension of a molten material to shape themicrolenses. The conventional method limits the maximum radius ofcurvature of each lens and hence the F-number of the lenses that can beproduced. The above method can be used to produce for example asphericlens shapes which give improved lens performance (less sphericalaberration).

[0084] A further advantage of fabricating a microlens array by the abovemethod is that a second optically functional surface or diffractiveoptical element, for example, can be formed on the surface of each ofthe lenses in the array at the same time as the lens itself is definedby use of a mould having the appropriate surface profile or diffractivestructure on its inner surface. Thus a profiled or combined refractivediffractive lens is produced. Such a combined lens performs a similaroptical function to an achromatic doublet lens (the combination of alens of negative dispersion with one with positive dispersion).

[0085] A further advantage of the above method is that large areas ofmicro relief arrays can be produced at once, in particular microlensarrays which are often required for use as display screens. Micro reliefarrays may comprise repeating sections of identical or different relieffeatures.

[0086] Due to the sub-micron resolution of the above method, microlenseswith small diameters and pitches may be produced.

[0087] A further advantage of the above method is that a set ofsubstantially identical structures may be produced. These may be used inassociated or unassociated arrangement.

[0088] In optical systems which use microlens arrays there is sometimesa requirement for an optical element which consists of two identicalmicrolens arrays placed back to back, separated by a fixed distancerelated to the focal length of the microlens array and with the twoarrays aligned relative to one another. An advantage of the above methodis that because the same mould can be used to form each array, the twoarrays will be identical. Accurate separation of the two arrays can beachieved by controlling the thickness of the intervening first layer andthe focal lengths of each array can be adjusted by changing therefractive index of the second array until the distance which separatesthe arrays is substantially the sum of their focal lengths. Furthermore,because the method can use an optically transparent flexible dispensinglayer, the second microlens array can be accurately aligned on the backof the first layer by viewing through the flexible dispensing layer.

[0089] The concept and applications of fabricating arrays of lightemitting diodes with integrated diffractive microlenses fabricated by adifferent method has recently been reported in “Arrays of light emittingdiodes with integrated diffractive microlenses for board-to-boardoptical interconnect applications: design, modelling and experimentalassessment”, B Dhoedt, P D Dobbelaere, J Blondelle, P V Daele, PDemeester, H Neefs, J V Campenhout, R Baets, Conference on Lasers andElectro-Optics (CLEO Europe 94), Amsterdam, August 28 to September 2,paper CTh164 (1994). The above method may also be used with atransparent embossing film to form MOEs onto the surface of a substratewhich already has semiconductor devices which emit or detect light (e.g.laser diodes, light emitting diodes, photodiodes and vertical cavitylasers) such that the MOE features are accurately aligned with thesemiconductor devices.

[0090] The above method may also be used to produce MREs which arealignment layers for liquid crystal cells. Some types of liquid crystalmaterial, in particular ferroelectric liquid crystals, require alignmentlayers in the cell to orient the liquid crystal in a certain way.Conventionally, the alignment layer can be produced by physicallypatterning the glass surface, for example by rubbing the surface in therequired direction. Alternatively, a thin layer of a material such asMgF₂ is evaporated onto the surface. The purpose of this alignment layeris to align the liquid crystal material with a small tilt relative tothe normal to the surface. By varying the angle of evaporation, theangle of the tilt can be varied. The current drawback of this method isthat the surface area is limited by the size of the evaporator'schamber. An advantage of the above process is that a larger surface areamay be structured using an embossed film prepared from several mastershims. Alternatively, alignment structures for liquid crystals may bemade for example in the form of a plurality of high aspect ratio MRE'sresembling relief “hairs” of the order of 200 nm high and 20 nm wide.Adventitiously, the ability to minimise the overlay is that there isless material covering the electrode which is used to apply an electricfield to the liquid crystal cell thereby potentially resulting in lowerswitching powers.

[0091] The present invention is illustrated in non-limiting manner byreference to the following figures.

[0092]FIG. 1 shows a section of the image produced by a 16×16 MOE beamarray generator.

[0093]FIG. 2 shows the variation in intensity with temperature for a 4×4beam array generator.

[0094]FIG. 3a shows a part of a nickel shim for preparing mould featuresin a flexible dispensing layer to be used to produce an MOE.

[0095]FIG. 3b shows a part of the MOE produced from the flexibledispensing layer prepared using the nickel shim shown in FIG. 3a.

[0096]FIGS. 4a and 4 b are SEMs showing a variety of surface reliefs.

[0097]FIG. 5 is an SEM of a relief feature in the form of a microlensarray.

[0098]FIG. 6 is Tencor Alpha-step surface profiling machine traceshowing overlayer thickness of an MOE.

[0099]FIG. 1 was produced from an MOE described in Example A.

[0100] In FIG. 2, line (1) represents the variation in temperature thatthe 4×4 beam array generator underwent as described in Example A. Line(2) represents the optical response of the equipment without any samplebeing present. Line (3) represents the optical response of the MOEfabricated on glass. Line (4) represents the optical response of the MOEfabricated on film. Line (5) represents the optical response when anarea of PET film with no MOE on it was illuminated.

[0101]FIG. 3a shows part of a nickel shim as used in Example B.

[0102]FIG. 3b shows part of microlens array produced according toExample B from a flexible dispensing layer in which the mould featureshad been formed using the nickel shim shown in FIG. 3a.

[0103]FIGS. 4a and 4 b show the various MREs produced in Example D.

[0104]FIG. 5 shows a hexagonal microlens array of 125 micron pitch and204 micron focal length in air as produced in Example E.

[0105]FIG. 6 was produced from an MOE described in Example F. In region(1) the polymer film was removed from the glass to provide a referencelevel.

[0106] The present invention is further illustrated in non-limitingmanner by reference to the following examples.

[0107] Preparation of a Flexible Dispensing Layer in the Form of anEmbossed Film

EXAMPLE 1.1

[0108] The following example describes the preparation of an embossedpolymer film having a release treated surface.

[0109] A wet coating of neat fluorinated dimethacrylate resin thickness20 m was applied to a 100 m thick polyester substrate (Melinex grade506). The coating was partially cured by exposing it to UV irradiationfor 2 s (whilst in air) from a Fisons F300 ultra violet lamp systemdelivering 300W/inch.

[0110] The coated polyester was then fed into a nip between a 400 mmdiameter steel roller which carried a nickel embossing shim containingsurface relief microstructures (e.g. 125 m pitch microlens arrays) and a150 mm diameter roller faced with silicone rubber of hardness 70 shore.The coated polyester entered the nip such that the coated side wasloaded against the shim. The nip load was controlled to 159 kg (350 lb)over a face width of 400 mm. The speed of the 400 mm diameter drum wasset to 3.3 cm.s⁻¹

[0111] On exiting the nip, the coated polyester and nickel shim passedthrough a UV source as described above which fully cured the coatingwhilst in contact with the shim to form the embossed polymer film. Theembossed film was then stripped away from the nickel shim and was thenbaked at 80 C for 16 hours in an oven.

[0112] A release layer of release material, Freekote FRP (DexterCorporation), was applied to the embossed film by washing with asolution of release material and then drying with compressed air. Thisprocess was repeated four times.

EXAMPLE 1.2

[0113] The following example describes the preparation of an embossedfilm which contains an internal release material.

[0114] A wet coat of 20 m was applied to 100 m polyester substrate(Melinex grade 506) from the following formulation:

[0115] 97.5 parts Ebercryl 150 (epoxy acrylate ex UCB Ltd.)

[0116] 2.5 parts Ebercryl 350 (silicone acrylate ex UCB Ltd.)

[0117] 20 parts LG156 (PMMA)

[0118] 2 parts Irgacure 651

[0119] mixed in solution 20% w/w in MEK. This resulted in a drythickness of 20 m.

[0120] This coated substrate was processed in the same way as describedin the Example 1.1, apart from the baking and the subsequent applicationof a release material.

EXAMPLE 2.1

[0121] The following example describes the preparation of MREs on arigid substrate using the previously prepared embossed film according toExample 1.1.

[0122] A rigid glass substrate was prepared by washing thoroughly in a30% Dekon 90 solution in water, a hot water rinse, an acetone wash andfinally a wash with isopropanol. The substrate was then dried in an ovenat 150 C. for 15 minutes.

[0123] The substrate was then positioned on a flat assembly bed andsecured by vacuum.

[0124] The assembly bed was provided with the means to traverse a 75 mmif diameter rubber covered nip roller along the length of the assemblybed, which forms an advancing nip region into which a UV source wasfocused.

[0125] An embossed film as described in Example 1.1 was placed face downon top of the glass substrate and anchored at one end with a singlesided adhesive tape.

[0126] A quantity of resin (LUXTRAK 0208), sufficient to fill the mouldfeatures in the embossed polymer was placed between the glass substrateand the embossed polymer as a bead across the direction of travel of theassembly bed and at the anchored end of the embossed film. Thetraversing roller was then positioned 3 mm before the bead of resin andhad a downward load of 40 kg across a face width of 80 mm applied.

[0127] The UV source was powered up and the nip roller was advanced atthe rate of 1 cm.s⁻¹ along the assembly bed across the embossedfilm/glass substrate. The resin was squeezed into the mould features andwas cured by the UV source. After curing the embossed polymer was peeledaway leaving the cured resin affixed to the glass substrate. 100%transfer was achieved although some witness marks of residual releasematerial were apparent.

EXAMPLE 2.2

[0128] The Example of 2.1 was repeated using the embossed film asprepared in Example 1.2, the LUXTRAK resin as described in Example 2.1and a fluorinated dimethacrylate resin of formulation:

[0129] Fluorodimethacrylate: 97 wt %

[0130] Photoinitiator (Irgacure 651): 2 wt %

[0131] Thermal initiator (Interox TBPEH): 1 wt %

[0132] 100% transfer was achieved for the LUXTRAK resin and about 80-90%for the fluoropolymer resin. There were no witness marks apparent.

EXAMPLE A

[0133] Using the method as described in Example 2.2, a number ofsynthetic MOEs, traditionally known as computer generated holograms,were fabricated to a depth 0.6 micron and a smallest lateral dimensionof 1.5 micron in LUXTRAK LCR 0208 on a glass substrate.

[0134] The chosen MOE was designed to produce an array of spots ofnearly equal optical power in the far-field behind the element when itwas illuminated with a laser beam of wavelength 670 nm. The laser beamwas derived from a diode laser but could have been produced by anothertype of laser source.

[0135] The fabrication of the master pattern was as described in“Synthetic diffractive elements for optical interconnects”, M RTaghizadeh and J Turunen, Optical Computing and Processing, vol 2 (4),p221-242, 1992. It consisted of a binary (2 level) surface reliefstructure produced in a quartz wafer. The diameter of the wafer waslarge enough to allow 12 MOEs, each of size 15 mm by 15 mm to be definedonto the one wafer surface. The surface of the quartz master wasrendered conducting by evaporating on a 10 nm thick layer of chromiumfollowed by a 60 run thick layer of silver. A nickel shim was then grownfrom the quartz master by an electroforming process.

[0136] The functions of each of the MOEs produced were 2×2, 4×2, 4×4,8×8, 8×16, 16×16 and 16×32 beam array generators. FIG. 1 shows thepattern produced when the 16×16 MOE was illuminated by the beam from adiode laser at a wavelength of 676 nm. This image was captured using aElectrophysics Micronviewer vidicon camera connected to an image capturesystem.

[0137] The intensity of one of the beams in the first order diffractionpattern of a 4×4 beam array generator MOE fabricated in LUXTRAK LCR 0208resin on a glass substrate as a function of temperature was compared tothat of the same MOE fabricated in urethane acrylate (Harcoss resin6217) on a “Melinex” film substrate. FIG. 2 shows the results of thisexperiment. The diffracted beam from the MOE fabricated on the filmvaried by up to 10% over the temperature range 25° C. to 85° C. Incomparison, the diffracted beam from the MOE fabricated on the glassvaried by only a few percent. A large variation was also observed whenthe beam passed through the film but outside of the patterned area. Thisindicates that the thermal mechanical behaviour of the substrate has astrong effect on the performance of the MOE.

EXAMPLE B

[0138] Example A was repeated except that the MOE fabricated acted as amicrolens array. In this example, the original master was produced bydirect electron beam writing of photoresist followed by dry etching ofthe pattern into quartz. The MOE contained 16 levels of surface relief(16 phase levels) so as to approximate more exactly a continuous surfaceprofile. The advantage of this is that the optical efficiency of the MOEis higher than the equivalent binary phase MOE. As a result of the extraphase levels the smallest lateral feature size in the surface relief wasabout 200 nm. This is significantly less than the smallest lateralfeature size on the binary surface relief. The UV embossing process usedto manufacture the MOEs has the capability to reproduce accurately thevery small features required. This is a significant advantage over othertypes of embossing methods (e.g. hot roll embossing or injectionmoulding). FIG. 3 shows for comparison an 800 micron aperture microlenson the nickel shim and the same lens formed in 2 micron thick urethaneacrylate resin (Harcross 6217) on 100 micron thick ICI Melinex film.

EXAMPLE C

[0139] Example A was repeated except that the Micro-Optical Element(MOE) fabricated was a surface relief diffraction grating of period 1.1m (smallest feature size 0.55 m) and depth 130 nm. The grating patternconsisted of an annulus of about 30 mm diameter and about 2 mm width.The surface of the MOE was coated with 70 nm of Aluminum by evaporationso as to render it highly reflective. The surface of the grating wasilluminated through the glass substrate using light from a He—Ne laserat 633 nm. The irradiance of light reflected from the grating into oneof the first diffraction orders was measured and compared to the amountof light reflected from an adjacent metallised area on the MOE where nograting was present. This ratio, also known as the efficiency, was foundto be 39±0.5%. The experiment was repeated using two other samplesmanufactured in the same way. Their diffraction efficiencies weremeasured to be 39±0.5% and 37±0.5% respectively. The efficiency isdirectly related to the accuracy with which the replication processreproduces the period and depth of the grating structure. Poorreplication of the surface relief results in efficiencies of less than10%.

[0140] The thickness of the overlayer was measured using a TencorAlpha-step surface profiling machine. The thickness was found to be 0.5m.

[0141] The experiment was repeated with the same grating pattern butusing a piece of the internal release coated “polymer shim” (describedin example 1.1) used in the manufacture of the replica on glass (i.e.the previous sample). Once again the sample was arranged so as to readout through the substrate. The diffraction efficiency was measured to be37%.

[0142] This experiment shows that there is no measurable reduction inefficiency due to the use of the polymer shim intermediate.

[0143] The experiment was repeated with the same grating pattern butusing a sample manufactured by coating a 2 m thick coating of urethaneacrylate (Harcross 6217) onto 175 m thick PET film (ICI MELINEX) and UVembossing. The diffraction efficiency was measured to be 36±0.5%. Thisexperiment shows that there is no reduction in efficiency by formulatingthe polymer shim material so as to contain an internal release agent.

[0144] The experiment was repeated with the same grating pattern butusing 0.5 mm thick polycarbonate sheet (LEXAN) as the substrate. Thediffraction efficiency was measured to be 36±0.5%. This experiment showsthat alternative rigid substrate materials can be used and that there isno significant reduction in efficiency from the resultant parts.

[0145] In all of the experiments above, the efficiency of the MOEproduced in this example is significantly larger than that measured fromthe same surface relief grating structure manufactured from the samemaster nickel shim by comparative techniques of hot embossing(efficiency 11%) and injection moulding (efficiency 4%).

EXAMPLE D

[0146] Using the method previously described in Example 1.1 an embossedfilm with a number of continuous surface relief microstructures wasfabricated. The structures included 12 micron high staircases, pyramidsof varying size, grooves, tracks, slopes, hemispherical structures andwells. FIG. 4 shows an SEM photograph of some of the structures formedaccording to Example 2.2 as LUXTRAK LCR 0208 on glass. Being able toprepare such deep relief features is an advantage because more phaseinformation can be impressed onto light which diffracts from it andhence the optical function of the relief features are enhanced.

EXAMPLE E

[0147] A nickel embossing shim was made by the following method:

[0148] A 100 mm square piece of glass was cleaned and dried. The glasssubstrate was placed in a vapour bath of Shipley Microposit primersolution for 2 minutes to improve the adhesion of the subsequentphotoresist layer. AZ4562 photoresist was spin coated onto the glasssubstrate at a speed of 2000 rpm for 20 s and the sample softbaked for10 minutes at 90° C. on a hotplate. The thickness of the photoresistlayer was measured to be 9.9 micron using a Tencor alpha-step machine.The sample was exposed for 35 s by contact through a photomaskcontaining a pattern of 125 micron pitch microlenses of diameter 120microns. The resist image was developed for 7.5 minutes in a 1:4 mixtureof AZ developer solution and water. The exposure and developmentconditions were chosen to ensure that all the photoresist had beenremoved between each microlens island. Finally, the microlenses wereformed by placing the sample onto a hotplate at 150° C. for 45 s. By sodoing, the resist material was caused to melt and surface tension drewthe resist islands into hemispherical microlenses.

[0149] The surface of the microlens sample was rendered conducting byevaporating thin chromium and silver layers onto it. A nickel master wasthen electroformed from the sample. The nickel master was used to growan embossing shim which was used to produce an embossed film asdescribed previously.

[0150] Using the laminating method previously described, a micro-opticallens array as shown in FIG. 5 was fabricated on a 2 mm thick glasssubstrate using fluorinated dimethacrylate resin.

EXAMPLE F

[0151] A microlens array was fabricated on a 1.1 mm thick borosilicateglass substrate (B270 glass) using the embossing shim whose preparationwas described in example E. The material used was Luxtrax LCR 0208 UVcure acrylate resin. The optical properties of the resin on glassreplica were measured so as to compare its optical performance to thatof the original melted photoresist lenses. The focal length over the 70mm by 70 mm area of the microlenses was found to be 204.4 m with astandard deviation of 1.5 m. The Strehl ratio was measured to be 0.82 (aStrehl ratio of 1 indicates diffraction limited performance). The lensshape was found to show only 0.55 of a wavelength deviation fromspherical when illuminated with light at 633 nm. These parameters arecomparable to those measured for similar melted photoresist microlenses,showing that the aberrations are introduced not during the replicationprocess, but are faithfully reproduced from aberrations present in themelted photoresist microlenses.

[0152] The thickness of the overlayer on this sample was measured usinga Tencor Alpha-step surface profiling machine. The trace obtained isshown in FIG. 6. The thickness was found to be less than 0.4 m. (Notethe relief structure height returns to the level which is bare glass(polymer has been removed to glass for reference purpose adjacent theboundary relief structure)).

EXAMPLE G

[0153] The Nickel embossing shim and method described in example E wasused to fabricate microlenses on the planar side of 25 mm diameterplano-convex glass lenses. In order to locate the lenses stably duringthe embossing/laminating process, they were mounted in an array in apolypropylene mounting plate with recesses machined into it using a toolof the same radius of curvature as the lenses. The use of thetransparent polymer shim enabled the embossed pattern to be accuratelycentred on the individual lenses. A further advantage of this method isthat the resultant part does not require any further cutting.

EXAMPLE H

[0154] The method described in Example E was used to fabricate amicrotens array on a 300 m thick glass substrate. A substrate of thisthickness was chosen so that the focal plane of the microlens arraywould coincide with the back surface of the glass substrate. The focallength of the lens array in glass is equal to the focal length in air(204 m) multiplied by the refractive index of the substrate (approx 1.5in this case). Small changes in the focal length could have been made bychanging the refractive index of the polymer resin by adding an indexmodifier to the formulation. However, this was not required in thisexample as the focal length in glass was approx 300 m.

[0155] Two samples were made and the lens arrays placed so that theiruncoated sides were in contact. Upon aligning the arrays so that themicrolenses were overlayed on top of each other, the combined lensarrays acted as a 1:1 relay lens and were able to image objects placedbeneath them. Thin glass is difficult to handle and breaks easilytherefore it would be difficult to have achieved this example by using aprocess which requires a high load.

EXAMPLE I

[0156] A microlens array was prepared in the same way as detailed inExample E. The microlens array was then positioned on a flat assemblybed and secured by vacuum such that the microlenses were on the topsurface. The assembly bed was provided with the means to traverse a 75mm diameter rubber covered nip roller along the length of the assemblybed, forming an advancing nip region into which a UV source could befocused.

[0157] A polyester laminating substrate, “Melinex” grade 400, was placedon top of the microlenses and anchored at one end with a single sidedadhesive tape. A quantity of resin with a different refractive index, inthis case a fluorodimethacrylate with 25 wt % added ethylene glycoldi-methacrylate, sufficient to encapsulate the microlenses, was placedbetween the microlenses and the polyester laminating substrate in a beadacross the direction of travel of the assembly bed and at the anchoredend of the laminate.

[0158] The traversing roller was then positioned 3 mm in front of thebead of resin and a downward load of 40 kg applied across a face widthof 80 mm. The UV source was switched on and the nip roller advanced atthe rate of 0.6 m.minute⁻¹ along the assembly bed across the laminate.The resin filled the cavities formed between the microlenses andlaminating substrate and was cured by the UV source. After curing thelaminating substrate was peeled away.

[0159] The purpose of this operation was to immerse the microlenses inthe higher index material so as to increase the focal length of themicrolenses compared to their focal length in air.

EXAMPLE J

[0160] An embossed film carrying a 500 micron pitch microlens array wasprepared using the method described in Example 1.1. The nickel embossingshim was selected to be a male so that the embossed film was female. Theembossed film was coated with ink so that ink was transferred to theintervening areas extending between the mould features. The embossedfilm was then used to prepare the microlenses as before. At the sametime as forming the nmicrolenses on the glass substrate, ink wastransferred to the unoccupied mglass surface between the microlenses.

[0161] The advantage of this process is that cross-talk between themicrolenses is reduced when the lenses are used in an optical system.

1. A composite micro relief element which comprises a) a first layer ofa first substrate, the first layer having a receptive surface capable ofretaining a relief forming polymer; (b) an overlay of a controlledthickness of the relief forming polymer over the receptive surface; and(c) at least one relief feature formed from the relief forming polymerand which protrudes above the overlay.
 2. A composite micro-opticalelement which comprises (a) a first layer of an optically transmissivefirst substrate having a first refractive index, the first layer havinga receptive surface capable of retaining an optically transmissiverelief forming polymer; (b) an overlay having an optically insignificanteffect, preferably having a maximum thickness of less than 1.5 m, of therelief forming polymer over the receptive surface, the relief formingpolymer having a second refractive index which is the same as ordifferent from the first refractive index; and (c) at least oneoptically active relief feature formed from the relief forming polymerand which protrudes above the overlay.
 3. An immersed compositemicrooptical element comprising (a) a first layer of an opticallytransmissive first substrate having a first refractive index, the firstlayer having a receptive surface capable of retaining an opticallytransmissive relief forming polymer; (b) an overlay having an opticallyinsignificant effect, preferably having a maximum thickness of less than1.5 m, of the relief forming polymer over the receptive surface, therelief forming optically transmissive polymer having a second refractiveindex which is the same as or different from the first refractive index;(c) at least one optically active relief feature formed from the reliefforming polymer and which protrudes above the overlay; and (d) a secondlayer of an optically transmissive second substrate having a thirdrefractive index which is superimposed upon the at least one opticallyactive relief feature and wherein not all of the first, second and thirdrefractive indices are the same.
 4. A composite element according to anyone of claims 1 to 3 which is a large area micro relief array comprisinga plurality of relief features, optionally comprising repeating sectionsof identical or different relief features.
 5. A composite elementaccording to any of claims 1 to 4 wherein the overlay of controlledthickness serves to planarise the receptive surface, preferably withthickness variation less than ±0.75 m.
 6. A composite element accordingto any of claims 1 to 5 wherein the relief feature(s) comprise one ormore continuous, stepped or otherwise profiled structures such as lens,straight or angled track or lateral, annular ring, straight or curveddiffraction grating, multiple faced (pyramidal) or other optical,fluidic, electrical or mechanical structure, and wherein the relieffeature(s) have an aspect ratio of up to
 20. 7. A composite elementaccording to any of claims 1 to 6 wherein the first layer, including anoptional support substrate, is planar, hollow or solid cylindrical, orcomprises a lens or other optical component, and is comprised of apolymer film, optically transparent material such as ZnSe or Germanium,silicon, high temperature resistant inorganic metal oxides or ceramicssuch as titania or (fused) silica and glass, or natural or syntheticpaper products such as wood pulp or synthetic card or paper.
 8. Acomposite element according to any of claims 1 to 7 wherein the firstlayer and/or the overlayer is selected or adapted to confer bonding,(internal) release, anti-reflection, heat dissipation, thermal expansionand/or thermo-optic, electrically conducting, optically modifying,reflection, or other suitable properties for example by means of choiceof first layer and/or overlayer material or of coating of the firstlayer and/or overlayer.
 9. A set of composite elements comprising anelement as defined in any of claims 1 to 8 which are substantiallyidentical, and are in associated or unassociated (for example comprisingtwo substantially identical micro lens arrays placed back to back)arrangement.
 10. A method of preparing a composite micro relief elementwhich comprises a) a first layer of a first substrate, the first layerhaving a receptive surface capable of retaining a relief formingpolymer; (b) an overlay of controlled thickness of the relief formingpolymer over the receptive surface; and (c) at least one relief featureformed from the relief forming polymer and which protrudes above theoverlay which method comprises (a) forming a line of contact between thereceptive surface and at least one mould feature formed in a flexibledispensing layer; (b) applying sufficient of a resin, capable of beingcured to form the relief forming polymer, to substantially fill the atleast one mould feature, along the line of contact; (c) progressivelycontacting the receptive surface with the flexible dispensing layer suchthat (1) the line of contact moves across the receptive surface; (2)sufficient of the resin is captured by the mould feature so as tosubstantially fill the mould feature; and (3) no more than a quantity ofresin capable of forming the overlay the line of contact; (d) curing theresin filling the at least one mould feature so as to form l-e at leastone relief feature; and, optionally, thereafter (e) releasing theflexible dispensing layer from the at least one relief feature.
 11. Amethod of preparing a composite micro-optical element which comprises(a) a first layer of an optically transmissive first substrate having afirst refractive index, the first layer having a receptive surfacecapable of retaining an optically transmissive relief forming polymer;(b) an overlay having an optically insignificant effect, preferablyhaving a maximum thickness of less than 1.5 m, of the relief formingpolymer over the receptive surface, the relief forming polymer having asecond refractive index which is the same as or different from the firstrefractive index; and (c) at least one optically active relief featureformed from the relief forming polymer and which protrudes above theoverlay which method comprises (a) forming a line of contact between thereceptive surface and at least one mould feature formed in a flexibledispensing layer; (b) applying sufficient of a resin, capable of beingcured to form the relief forming polymer, to substantially fill the atleast one mould feature, along the line of contact; (c) progressivelycontacting the receptive surface with the flexible dispensing layer suchthat (1) the line of contact moves across the receptive surface; (2)sufficient of the resin is captured by the mould feature so as tosubstantially fill the mould feature; and (3) no more than a quantity ofresin capable of forming the overlay passes the line of contact; (d)curing the resin filling the at least one mould feature so as to formthe at least one optically active relief feature; and, optionally,thereafter (e) releasing the flexible dispensing layer from the at leastone optically active relief feature.
 12. A method according to either ofclaims 10 or 11 wherein the flexible dispensing layer comprises apolymer film into which the mould features have been embossed,preferably is transparent to UV light, has high quality surface releaseproperties and is capable of remaining dimensionally sound during themoulding process.
 13. A method according to any of claims 10 to 12wherein a flexible dispensing layer comprising an embossed film isformed by (a) forming a master pattern having a contoured metallisedsurface which conforms to the required relief structure, (b)electroforming a layer of a first metal onto the metallised surface toform a metal master, (c) releasing the metal master from the masterpattern, (d) repeating the electroforming process to form a metalembossing master shim and (e) embossing the relief structure into apolymer film so as to provide an embossed film having the desired mouldfeatures.
 14. A method according to any of claims 10 to 13 whereinpressure is applied along the line of contact by means, of using anadvancing bar or flexible blade under a compressive load which may bedrawn along the surface, or using a roller under a compressive loadwhich may be rotated along the surface.
 15. An apparatus for use in amethod according to any of claims 10 to 14 comprising a roller havingmould features suitable for the manufacture of an element according toany of claims 1 to
 10. 16. An embossed polymer film or flexibledispensing layer according to any of claims 10 to 13, optionally securedto the surface of a roller according to claim 14 or 15 and comprisingmould features suitable for the manufacture of an element according toany of claims 1 to
 9. 17. An element or method as hereinbefore definedwith reference to any of claims 1 to 16 for use in micro-optical,micro-fluidic, micro-electrical or micro-mechanical application.
 18. Anelement, method or apparatus as hereinbefore defined with reference tothe description or examples.